Diversity in the regulatory B-subunits of protein phosphatase 2A: identification of a novel isoform highly expressed in brain

Mapping the substrate landscape of protein phosphatase 2A catalytic subunit PPP2CA

Protein phosphatase 2A (PP2A) is an essential Ser/Thr phosphatase. The PP2A holoenzyme complex comprises a scaffolding (A), regulatory (B), and catalytic (C) subunit, with PPP2CA being the principal catalytic subunit. The full scope of PP2A substrates in cells remains to be defined. To address this, we employed dTAG proteolysis-targeting chimeras to efficiently and selectively degrade dTAG-PPP2CA in homozygous knock-in HEK293 cells. Unbiased global phospho-proteomics identified 2,204 proteins with significantly increased phosphorylation upon dTAG-PPP2CA degradation, implicating them as potential PPP2CA substrates. A vast majority of these are novel. Bioinformatic analyses revealed involvement of the potential PPP2CA substrates in spliceosome function, cell cycle, RNA transport, and ubiquitin-mediated proteolysis. We identify a pSP/pTP motif as a predominant target for PPP2CA and confirm some of our phospho-proteomic data with immunoblotting. We provide an in-depth atlas of potential PPP2CA substrates and establish targeted degradation as a robust tool to unveil phosphatase substrates in cells.

ATM phosphorylates PP2A subunit A resulting in nuclear export and spatiotemporal regulation of the DNA damage response

Ataxia telangiectasia mutated (ATM) is a serine-threonine protein kinase and important regulator of the DNA damage response (DDR). One critical ATM target is the structural subunit A (PR65-S401) of protein phosphatase 2A (PP2A), known to regulate diverse cellular processes such as mitosis and cell growth as well as dephosphorylating many proteins during the recovery from the DDR. We generated mouse embryonic fibroblasts expressing PR65-WT, -S401A (cannot be phosphorylated), and -S401D (phospho-mimetic) transgenes. Significantly, S401 mutants exhibited extensive chromosomal aberrations, impaired DNA double-strand break (DSB) repair and underwent increased mitotic catastrophe after radiation. Both S401A and the S401D cells showed impaired DSB repair (nonhomologous end joining and homologous recombination repair) and exhibited delayed DNA damage recovery, which was reflected in reduced radiation survival. Furthermore, S401D cells displayed increased ERK and AKT signaling resulting in enhanced growth rate further underscoring the multiple roles ATM-PP2A signaling plays in regulating prosurvival responses. Time-lapse video and cellular localization experiments showed that PR65 was exported to the cytoplasm after radiation by CRM1, a nuclear export protein, in line with the very rapid pleiotropic effects observed. A putative nuclear export sequence (NES) close to S401 was identified and when mutated resulted in aberrant PR65 shuttling. Our study demonstrates that the phosphorylation of a single, critical PR65 amino acid (S401) by ATM fundamentally controls the DDR, and balances DSB repair quality, cell survival and growth by spatiotemporal PR65 nuclear-cytoplasmic shuttling mediated by the nuclear export receptor CRM1.

In Silico Analysis to Identify Molecular Targets for Chemicals of Concern: The Case Study of Flocoumafen, an Anticoagulant Pesticide

Rodenticides are pesticides used worldwide, with little information available regarding health consequences in wildlife and humans. The aim of the present study was to use virtual screening to identify potential targets for flocoumafen, a superwarfarin rodenticide. Blind docking of more than 841 human proteins was carried out employing AutoDock Vina. The strength of the ligand interaction with the proteins was quantified based on the binding affinity score (kcal/mol). Results indicate that flocoumafen could be a promiscuous ligand for diversity of cellular protein targets. The best complexes were obtained for prostaglandin F synthase (-14.2 kcal/mol) and serum albumin (-14.0 kcal/mol) followed by glucocorticoid receptor 2, matrix metalloproteinase-9, nuclear receptor ROR-alpha, and activin receptor type-1, all with values equal or better than -13.5 kcal/mol. Docking method validation based on the root-mean-square deviation showed that flocoumafen had good capability to predict corresponding co-crystallized poses; and molecular dynamics simulations suggested the complex with greater binding affinity was thermodynamically stable. Protein-protein interaction networks built with main protein targets revealed that protein kinase B (AKT1), ribosomal protein S6 kinase B1 (RPS6KB1), phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA), retinoid X receptor alpha (RXRA), and protein phosphatase 2 catalytic subunit alpha (PPP2CA) were major hub proteins, whereas the gene ontology analysis reported that cellular response to endogenous stimulus, protein binding, and the TOR complex were the biological processes, molecular function, and cell component enrichments, respectively. These results should motivate more ecotoxicity testing for flocoumafen and other superwarfarins, as well as precautionary legislation to minimize exposure to these highly toxic chemicals. Environ Toxicol Chem 2021;40:2034-2043. © 2021 SETAC.

Protein phosphatase 2A has an essential role in promoting thymocyte survival during selection

The development of thymocytes to mature T cells in the thymus is tightly controlled by cellular selection, in which only a small fraction of thymocytes equipped with proper quality of TCRs progress to maturation. It is pivotal to protect the survival of the few T cells, which pass the selection. However, the signaling events, which safeguard the cell survival in thymus, are not totally understood. In this study, protein Ser/Thr phosphorylation in thymocytes undergoing positive selection is profiled by mass spectrometry. The results revealed large numbers of dephosphorylation changes upon T cell receptor (TCR) activation during positive selection. Subsequent substrate analysis pinpointed protein phosphatase 2A (PP2A) as the enzyme responsible for the dephosphorylation changes in developing thymocytes. PP2A catalytic subunit α (Ppp2ca) deletion in the T cell lineage in Ppp2ca ^flox/flox-Lck-Cre mice (PP2A cKO) displayed dysregulated dephosphorylation of apoptosis-related proteins in double-positive (DP) cells and caused substantially decreased numbers of DP CD4⁺ CD8⁺ cells. Increased levels of apoptosis in PP2A cKO DP cells were found to underlie aberrant thymocyte development. Finally, the defective thymocyte development in PP2A cKO mice could be rescued by either Bcl2 transgene expression or by p53 knockout. In summary, our work reveals an essential role of PP2A in promoting thymocyte development through the regulation of cell survival.

PPP2R2D, a regulatory subunit of protein phosphatase 2A, promotes gastric cancer growth and metastasis via mechanistic target of rapamycin activation

Protein phosphatase 2A (PP2A) is an essential serine/threonine protein phosphatase that regulates the basic activities of eukaryotes by dephosphorylating its substrates. The function and substrate specificity of PP2A are generally determined by its regulatory subunits. In the present study, the clinical significance and roles of PPP2R2D, one of the regulatory subunits of PP2A, were demonstrated in gastric cancer (GC) carcinogenesis. Through a tissue microarray and quantitative polymerase chain reaction analysis, it was demonstrated that PPP2R2D was commonly upregulated in GC samples. This upregulation was positively correlated with the patients' tumor stage (P<0.01), T classification (P<0.01) and N classification (P=0.01). Furthermore, a high expression of PPP2R2D was closely associated with poor prognosis of patients. Knockdown of PPP2R2D significantly inhibited the proliferation and migration of GC cells in vitro, as well as the tumorigenicity and metastasis in vivo in an animal GC model. By contrast, overexpression of PPP2R2D promoted GC cell proliferation and migration in vitro. The analysis of underlying mechanisms indicated that PPP2R2D silencing decreased the phosphorylation level of mechanistic target of rapamycin (mTOR), thereby implicating that PPP2R2D is involved in the regulation of mTOR activity during tumorigenesis. Thus, the findings of the present study suggested that PPP2R2D may serve as a potential oncogene in GC and as a novel target for therapeutic strategies against this disease.

Genome-Wide Screen for New Components of the Drosophila melanogaster Torso Receptor Tyrosine Kinase Pathway

Patterning of the Drosophila embryonic termini by the Torso (Tor) receptor pathway has long served as a valuable paradigm for understanding how receptor tyrosine kinase signaling is controlled. However, the mechanisms that underpin the control of Tor signaling remain to be fully understood. In particular, it is unclear how the Perforin-like protein Torso-like (Tsl) localizes Tor activity to the embryonic termini. To shed light on this, together with other aspects of Tor pathway function, we conducted a genome-wide screen to identify new pathway components that operate downstream of Tsl. Using a set of molecularly defined chromosomal deficiencies, we screened for suppressors of ligand-dependent Tor signaling induced by unrestricted Tsl expression. This approach yielded 59 genomic suppressor regions, 11 of which we mapped to the causative gene, and a further 29 that were mapped to <15 genes. Of the identified genes, six represent previously unknown regulators of embryonic Tor signaling. These include twins (tws), which encodes an integral subunit of the protein phosphatase 2A complex, and α-tubulin at 84B (αTub84B), a major constituent of the microtubule network, suggesting that these may play an important part in terminal patterning. Together, these data comprise a valuable resource for the discovery of new Tor pathway components. Many of these may also be required for other roles of Tor in development, such as in the larval prothoracic gland where Tor signaling controls the initiation of metamorphosis.

Regulatory roles of phosphorylation in model and pathogenic fungi

Over the past 20 years, considerable advances have been made toward our understanding of how post-translational modifications affect a wide variety of biological processes, including morphology and virulence, in medically important fungi. Phosphorylation stands out as a key molecular switch and regulatory modification that plays a critical role in controlling these processes. In this article, we first provide a comprehensive and up-to-date overview of the regulatory roles that both Ser/Thr and non-Ser/Thr kinases and phosphatases play in model and pathogenic fungi. Next, we discuss the impact of current global approaches that are being used to define the complete set of phosphorylation targets (phosphoproteome) in medically important fungi. Finally, we provide new insights and perspectives into the potential use of key regulatory kinases and phosphatases as targets for the development of novel and more effective antifungal strategies.

The Human Adenovirus Type 5 E4orf4 Protein Targets Two Phosphatase Regulators of the Hippo Signaling Pathway

When expressed alone at high levels, the human adenovirus E4orf4 protein exhibits tumor cell-specific p53-independent toxicity. A major E4orf4 target is the B55 class of PP2A regulatory subunits, and we have shown recently that binding of E4orf4 inhibits PP2A(B55) phosphatase activity in a dose-dependent fashion by preventing access of substrates (M. Z. Mui et al., PLoS Pathog 9:e1003742, 2013, http://dx.doi.org/10.1371/journal.ppat.1003742). While interaction with B55 subunits is essential for toxicity, E4orf4 mutants exist that, despite binding B55 at high levels, are defective in cell killing, suggesting that other essential targets exist. In an attempt to identify additional targets, we undertook a proteomics approach to characterize E4orf4-interacting proteins. Our findings indicated that, in addition to PP2A(B55) subunits, ASPP-PP1 complex subunits were found among the major E4orf4-binding species. Both the PP2A and ASPP-PP1 phosphatases are known to positively regulate effectors of the Hippo signaling pathway, which controls the expression of cell growth/survival genes by dephosphorylating the YAP transcriptional coactivator. We find here that expression of E4orf4 results in hyperphosphorylation of YAP, suggesting that Hippo signaling is affected by E4orf4 interactions with PP2A(B55) and/or ASPP-PP1 phosphatases. Furthermore, knockdown of YAP1 expression was seen to enhance E4orf4 killing, again consistent with a link between E4orf4 toxicity and inhibition of the Hippo pathway. This effect may in fact contribute to the cancer cell specificity of E4orf4 toxicity, as many human cancer cells rely heavily on the Hippo pathway for their enhanced proliferation.

IMPORTANCE

The human adenovirus E4orf4 protein has been known for some time to induce tumor cell-specific death when expressed at high levels; thus, knowledge of its mode of action could be of importance for development of new cancer therapies. Although the B55 form of the phosphatase PP2A has long been known as an essential E4orf4 target, genetic analyses indicated that others must exist. To identify additional E4orf4 targets, we performed, for the first time, a large-scale affinity purification/mass spectrometry analysis of E4orf4 binding partners. Several additional candidates were detected, including key regulators of the Hippo signaling pathway, which enhances cell viability in many cancers, and results of preliminary studies suggested a link between inhibition of Hippo signaling and E4orf4 toxicity.

Visualization of subunit interactions and ternary complexes of protein phosphatase 2A in mammalian cells

Protein phosphatase 2A (PP2A) is a ubiquitous phospho-serine/threonine phosphatase that controls many diverse cellular functions. The predominant form of PP2A is a heterotrimeric holoenzyme consisting of a scaffolding A subunit, a variable regulatory B subunit, and a catalytic C subunit. The C subunit also associates with other interacting partners, such as α4, to form non-canonical PP2A complexes. We report visualization of PP2A complexes in mammalian cells. Bimolecular fluorescence complementation (BiFC) analysis of PP2A subunit interactions demonstrates that the B subunit plays a key role in directing the subcellular localization of PP2A, and confirms that the A subunit functions as a scaffold in recruiting the B and C subunits to form a heterotrimeric holoenzyme. BiFC analysis also reveals that α4 promotes formation of the AC core dimer. Furthermore, we demonstrate visualization of specific ABC holoenzymes in cells by combining BiFC and fluorescence resonance energy transfer (BiFC-FRET). Our studies not only provide direct imaging data to support previous biochemical observations on PP2A complexes, but also offer a promising approach for studying the spatiotemporal distribution of individual PP2A complexes in cells.

Protein phosphatase 2A (PP2A) regulatory subunits ParA and PabA orchestrate septation and conidiation and are essential for PP2A activity in Aspergillus nidulans

Protein phosphatase 2A (PP2A) is a major intracellular protein phosphatase that regulates multiple aspects of cell growth and metabolism. Different activities of PP2A and subcellular localization are determined by its regulatory subunits. Here we identified and characterized the functions of two protein phosphatase regulatory subunit homologs, ParA and PabA, in Aspergillus nidulans. Our results demonstrate that ParA localizes to the septum site and that deletion of parA causes hyperseptation, while overexpression of parA abolishes septum formation; this suggests that ParA may function as a negative regulator of septation. In comparison, PabA displays a clear colocalization pattern with 4',6-diamidino-2-phenylindole (DAPI)-stained nuclei, and deletion of pabA induces a remarkable delayed-septation phenotype. Both parA and pabA are required for hyphal growth, conidiation, and self-fertilization, likely to maintain normal levels of PP2A activity. Most interestingly, parA deletion is capable of suppressing septation defects in pabA mutants, suggesting that ParA counteracts PabA during the septation process. In contrast, double mutants of parA and pabA led to synthetic defects in colony growth, indicating that ParA functions synthetically with PabA during hyphal growth. Moreover, unlike the case for PP2A-Par1 and PP2A-Pab1 in yeast (which are negative regulators that inactivate the septation initiation network [SIN]), loss of ParA or PabA fails to suppress defects of temperature-sensitive mutants of the SEPH kinase of the SIN. Thus, our findings support the previously unrealized evidence that the B-family subunits of PP2A have comprehensive functions as partners of heterotrimeric enzyme complexes of PP2A, both spatially and temporally, in A. nidulans.

Phosphatase: PP2A structural importance, regulation and its aberrant expression in cancer

Protein Phosphatase 2A (PP2A) is an important and ubiquitously expressed serine threonine phosphatase and regulates the function by dephosphorylating many critical cellular molecules like Akt, p53, c-Myc and β-catenin. It plays a critical role in cellular processes, such as cell proliferation, signal transduction and apoptosis. Structurally, it is multifarious as it is composed of catalytic, scaffold and regulatory subunits. The catalytic and scaffold subunits have two isoforms and the regulatory subunit has four different families containing different isoforms. The regulatory subunit is the most diverse with temporal and spatial specificity. PP2A undergoes post-translational modifications (i.e. phosphorylation and methylation), which in turn, regulates its enzymatic activity. Aberrant expression, mutations and somatic alterations of the PP2A scaffold and regulatory subunits have been observed in various human malignancies, including lung, breast, skin and colon cancer, highlighting its role as a 'tumor suppressor'. This review is focused on the structural complexity of serine/threonine phosphatase PP2A and summarizes its expression pattern in cancer. Additionally, the PP2A interacting and regulatory proteins and substrates are also discussed. Finally, the mouse models developed to understand the biological role of PP2A subunits in an in vivo model system are also reviewed in this article.

Structure, regulation, and pharmacological modulation of PP2A phosphatases

Protein phosphatases of the type 2A family (PP2A) represent a major fraction of cellular Ser/Thr phosphatase activity in any given human tissue. In this review, we describe how the holoenzymic nature of PP2A and the existence of several distinct PP2A composing subunits allow for the generation of multiple structurally and functionally different PP2A complexes, explaining why PP2A is involved in the regulation of so many diverse cell biological and physiological processes. Moreover, in human disease, most notably in several cancers and Alzheimer's Disease, PP2A expression and/or activity have been found significantly decreased, underscoring its important functions as a major tumor suppressor and tau phosphatase. Hence, several recent preclinical studies have demonstrated that pharmacological restoration of PP2A activity, as well as pharmacological PP2A inhibition, under certain conditions, may be of significant future therapeutic value.

Bioengineered protein phosphatase 2A: update on need

Harmful algal blooms caused by phytoplankton can occur in all aquatic environments. Some of the algae present in these blooms are capable of producing extremely potent toxins. Due to climate change and eutrophication, harmful algal blooms are increasing on a global scale. One kind of toxin producing algae are those that produce okadaic acid, its derivatives (dinophysistoxin-1 and 2), and microcystins. These toxins are potent inhibitors of protein phosphatase 2A, so this protein is used to detect the mentioned toxins in natural samples. Originally protein phosphatase 2A purified from animal tissues was used, but enzyme activity and stability fluctuations prevented the use of the enzyme in detection kits. Expression of the enzyme as a recombinant protein provided a solution to this problem. For this purpose, several strategies have been followed. We evaluated the activity, specificity and stability of the human protein phosphatase 2A catalytic subunit α expressed in insect larvae and showed that this expression system can be a reliable source of high quantities of stable enzyme.

The biogenesis of active protein phosphatase 2A holoenzymes: a tightly regulated process creating phosphatase specificity

Protein phosphatase type 2A (PP2A) enzymes constitute a large family of Ser/Thr phosphatases with multiple functions in cellular signaling and physiology. The composition of heterotrimeric PP2A holoenzymes, resulting from the combinatorial assembly of a catalytic C subunit, a structural A subunit, and regulatory B-type subunit, provides the essential determinants for substrate specificity, subcellular targeting, and fine-tuning of phosphatase activity, largely explaining why PP2A is functionally involved in so many diverse physiological processes, sometimes in seemingly opposing ways. In this review, we highlight how PP2A holoenzyme biogenesis and enzymatic activity are controlled by a sophisticatedly coordinated network of five PP2A modulators, consisting of α4, phosphatase 2A phosphatase activator (PTPA), leucine carboxyl methyl transferase 1 (LCMT1), PP2A methyl esterase 1 (PME-1) and, potentially, target of rapamycin signaling pathway regulator-like 1 (TIPRL1), which serve to prevent promiscuous phosphatase activity until the holoenzyme is completely assembled. Likewise, these modulators may come into play when PP2A holoenzymes are disassembled following particular cellular stresses. Malfunctioning of these cellular control mechanisms contributes to human disease. The potential therapeutic benefits or pitfalls of interfering with these regulatory mechanisms will be briefly discussed.

Identification and functional analysis of variant haplotypes in the 5'-flanking region of protein phosphatase 2A-Bδ gene

Serine-threonine protein phosphatase 2A (PP2A) is a trimeric holoenzyme that plays an integral role in the regulation of cell growth, differentiation, and apoptosis. The substrate specificity and (sub)cellular localization of the PP2A holoenzymes are highly regulated by interaction with a family of regulatory B subunits (PP2A-Bs). The regulatory subunit PP2A-B/PR55δ (PP2A-Bδ) is involving in the dephosphorylation of PP2A substrates and is crucial for controlling entry into and exit from mitosis. The molecular mechanisms involved in the regulation of expression of PP2A-Bδ gene (PPP2R2D) remain largely unknown. To explore genetic variations in the 5′-flanking region of PPP2R2D gene as well as their frequent haplotypes in the Han Chinese population and determine whether such variations have an impact on transcriptional activity, DNA samples were collected from 70 healthy Chinese donors and sequenced for identifying genetic variants in the 5′-flanking region of PPP2R2D. Four genetic variants were identified in the 1836 bp 5′-flanking region of PPP2R2D. Linkage disequilibrium (LD) patterns and haplotype profiles were constructed for the genetic variants. Using serially truncated human PPP2R2D promoter luciferase constructs, we found that a 601 bp (−540 nt to +61 nt) fragment constitutes the core promoter region. The subcloning of individual 5′-flanking fragment revealed the existence of three haplotypes in the distal promoter of PPP2R2D. The luciferase reporter assay showed that different haplotypes exhibited distinct promoter activities. The EMSA revealed that the −462 G>A variant influences DNA-protein interactions involving the nuclear factor 1 (NF1). In vitro reporter gene assay indicated that cotransfection of NF1/B expression plasmid could positively regulate the activity of PPP2R2D proximal promoter. Introduction of exogenous NF1/B expression plasmid further confirmed that the NF1 involves in the regulation of PPP2R2D gene expression. Our findings suggest that functional genetic variants and their haplotypes in the 5′-flanking region of PPP2R2D are critical for transcriptional regulation of PP2A-Bδ.

Spinocerebellar ataxia type 12

SCA12 is a late-onset, autosomal dominant, slowly progressive disorder. Action tremor is the usual presenting sign. Subsequent development of ataxia and hyperreflexia suggests spinocerebellar ataxia. In the index SCA12 kindred, which resides in North America and is of German ancestry, parkinsonism, anxiety, depression, and cognitive dysfunction are not uncommon. SCA12 is linked to a CAG repeat expansion mutation in exon 7 of PPP2R2B, a gene that encodes Bβ, a regulatory subunit of protein phosphatase 2A (PP2A). CAG repeats number 7-28 in normal individuals and 55-78 in SCA12 patients. The mechanism by which this mutation leads to SCA12 has not been determined. The CAG expansion in PPP2R2B has promoter function in vitro. CAG length correlates with increased Bβ expression. There is no evidence that this CAG expansion results in polyglutamine production. In addition to the North. American SCA12 kindred, multiple SCA12 families have been found in Northern India that are not related to the index SCA12 kindred. SCA12 has been reported, rarely, in Singapore and China. Action tremor, anxiety, and depression in SCA12 have responded to usual treatments for these disorders. SCA12 may be considered in patients who present with action tremor and later develop signs of cerebellar and cortical dysfunction.

The substrate of Greatwall kinase, Arpp19, controls mitosis by inhibiting protein phosphatase 2A

Initiation and maintenance of mitosis require the activation of protein kinase cyclin B-Cdc2 and the inhibition of protein phosphatase 2A (PP2A), which, respectively, phosphorylate and dephosphorylate mitotic substrates. The protein kinase Greatwall (Gwl) is required to maintain mitosis through PP2A inhibition. We describe how Gwl activation results in PP2A inhibition. We identified cyclic adenosine monophosphate-regulated phosphoprotein 19 (Arpp19) and α-Endosulfine as two substrates of Gwl that, when phosphorylated by this kinase, associate with and inhibit PP2A, thus promoting mitotic entry. Conversely, in the absence of Gwl activity, Arpp19 and α-Endosulfine are dephosphorylated and lose their capacity to bind and inhibit PP2A. Although both proteins can inhibit PP2A, endogenous Arpp19, but not α-Endosulfine, is responsible for PP2A inhibition at mitotic entry in Xenopus egg extracts.

Genetic analysis of B55alpha/Cdc55 protein phosphatase 2A subunits: association with the adenovirus E4orf4 protein

The human adenovirus E4orf4 protein is toxic in both human tumor cells and Saccharomyces cerevisiae. Previous studies indicated that most of this toxicity is dependent on an interaction of E4orf4 protein with the B55 class of regulatory subunits of protein phosphatase 2A (PP2A) and in yeast with the B55 homolog Cdc55. We have found previously that E4orf4 inhibits PP2A activity against at least some substrates. In an attempt to understand the mechanism of this inhibition, we used a genetic approach to identify residues in the seven-bladed β-propeller proteins B55α and Cdc55 required for E4orf4 binding. In both cases, amino-terminal polypeptides composed only of blade 1 and at least part of blade 2 were found to bind E4orf4 and overexpression blocked E4orf4 toxicity in yeast. Furthermore, certain amino acid substitutions in blades 1 and 2 within full-length B55α and Cdc55 resulted in loss of E4orf4 binding. Recent mutational analysis has suggested that segments of blades 1 and 2 present on the top face of B55α form part of the "substrate-binding groove." Additionally, these segments are in close proximity to the catalytic C subunit of the PP2A holoenzyme. Thus, our results are consistent with the hypothesis that E4orf4 binding could affect the access of substrates, resulting in the failure to dephosphorylate some PP2A substrates.

SV40 small T antigen and PP2A phosphatase in cell transformation

The SV40 early region protein, SV40 small t antigen, promotes cell transformation through negative regulation of the protein phosphatase 2A (PP2A) family of serine-threonine phosphatases. More recently, reduced levels of PP2A activity have been found in different types of human cancer. This occurs either through inactivating mutations of PP2A structural subunits, or by upregulation of the cellular PP2A inhibitors, CIP2A and SET. Several distinct PP2A complexes have been identified that contribute directly to tumor suppression by regulating specific phosphorylation events. These studies provide us with new insights into the role of protein phosphatases in cancer initiation and maintenance.

Protein phosphatase 2A regulatory subunits and cancer

The serine/threonine protein phosphatase (PP2A) is a trimeric holoenzyme that plays an integral role in the regulation of a number of major signaling pathways whose deregulation can contribute to cancer. The specificity and activity of PP2A are highly regulated through the interaction of a family of regulatory B subunits with the substrates. Accumulating evidence indicates that PP2A acts as a tumor suppressor. In this review we summarize the known effects of specific PP2A holoenzymes and their roles in cancer relevant pathways. In particular we highlight PP2A function in the regulation of MAPK and Wnt signaling.

Multiple pathways regulated by the tumor suppressor PP2A in transformation

Reversible protein phosphorylation plays a central role in regulating intracellular signaling. Dysregulation of the mechanisms that regulate phosphorylation plays a direct role in cancer initiation and maintenance. Although abundant evidence supports the role of kinase oncogenes in cancer development, recent work has illuminated the role of specific protein phosphatases in malignant transformation. Protein phosphatase 2A (PP2A) is the major serine-threonine phosphatase in mammalian cells. Inactivation of PP2A by viral oncoproteins, mutation of specific subunits or overexpression of endogenous inhibitors contributes to cell transformation by regulating specific phosphorylation events. Here, we review recent progress in our understanding of how PP2A regulates mitogenic signaling pathways in cancer pathogenesis and how PP2A activity is modulated in human cancers.

Regulation of phosphorylation of Thr-308 of Akt, cell proliferation, and survival by the B55alpha regulatory subunit targeting of the protein phosphatase 2A holoenzyme to Akt

Akt is a protein serine/threonine kinase that is involved in the regulation of diverse cellular processes. Phosphorylation of Akt at regulatory residues Thr-308 and Ser-473 leads to its full activation. The protein phosphatase 2A (PP2A) has long been known to negatively regulate Akt activity. The PP2A holoenzyme consists of the structural subunit (A), catalytic subunit (C), and a variable regulatory subunit (B). Here we report the identification of the specific B regulatory subunit that targets the PP2A holoenzyme to Akt. We found endogenous association of PP2A AB55C holoenzymes with Akt by co-immunoprecipitation analyses in pro-lymphoid FL5.12 cells. Akt was shown to associate with ectopically expressed B55alpha subunit in NIH3T3 cells. The direct interaction between B55alpha subunit and Akt was confirmed using in vitro pulldown analyses. Intriguingly, we found that overexpression of B55alpha subunit significantly impaired phosphorylation at Thr-308, but to a lesser extent at Ser-473 of Akt in both FL5.12 and NIH3T3 cells. Concomitantly, phosphorylation of a subset of Akt substrates, including FoxO3a, was substantially decreased by B55alpha overexpression in these cells. Silencing of B55alpha expression markedly increased phosphorylation at Thr-308 but not at Ser-473 in both FL5.12 cells and NIH3T3 cells. Consistently, PP2A AB55alphaC holoenzymes preferentially dephosphorylated phospho-Thr-308 rather than phospho-Ser-473 in in vitro dephosphorylation assays. Furthermore, B55alpha overexpression retarded proliferation of NIH3T3 cells, and knockdown of B55alpha expression increased survival of FL5.12 cells upon interleukin-3 deprivation. Together, our data demonstrate that B55alpha-dependent targeting of the PP2A holoenzyme to Akt selectively regulates Akt phosphorylation at Thr-308 to regulate cell proliferation and survival.

Functional interaction of Aurora-A and PP2A during mitosis

Entry into mitosis is a highly regulated process, promoted by the activated Cyclin B1/Cdk1 complex. Activation of this complex is controlled, in part, by the protein kinase Aurora-A, which is a member of a multigenic serine/threonine kinase family. In normal cells, Aurora-A activity is regulated, at least in part, by degradation through the APC-ubiquitin-proteasome pathway. It has recently been proposed that, in Xenopus, Aurora-A degradation can be inhibited by phosphorylation. It would thus be expected that a phosphatase activity would release this blockade at the end of mitosis. Here, we have shown that the protein phosphatase PP2A and Aurora-A are colocalized at the cell poles during mitosis in human cells and interact within the same complex. Using the PP2A inhibitor okadaic acid and an RNAi approach, we have shown that this interaction is functional within the cell. PP2A/Aurora-A interaction is promoted by an S51D mutation in Aurora-A and inhibited by a phosphomimetic peptide centered around Aurora-A S51, thereby strongly suggesting that PP2A controls Aurora-A degradation by dephosphorylating serine 51 in the A box of the human enzyme.

The B' protein phosphatase 2A regulatory subunit well-rounded regulates synaptic growth and cytoskeletal stability at the Drosophila neuromuscular junction

Synaptic growth is essential for the development and plasticity of neural circuits. To identify molecular mechanisms regulating synaptic growth, we performed a gain-of-function screen for synapse morphology mutants at the Drosophila neuromuscular junction (NMJ). We isolated a B' regulatory subunit of protein phosphatase 2A (PP2A) that we have named well-rounded (wrd). Neuronal overexpression of wrd leads to overgrowth of the synaptic terminal. Endogenous Wrd protein is present in the larval nervous system and muscle and is enriched at central and neuromuscular synapses. wrd is required for normal synaptic development; in its absence, there are fewer synaptic boutons and there is a decrease in synaptic strength. wrd functions presynaptically to promote normal synaptic growth and postsynaptically to maintain normal levels of evoked transmitter release. In the absence of wrd, the presynaptic cytoskeleton is abnormal, with an increased proportion of unbundled microtubules. Reducing PP2A enzymatic activity also leads to an increase in unbundled microtubules, an effect enhanced by reducing wrd levels. Hence, wrd promotes the function of PP2A and is required for normal cytoskeletal organization, synaptic growth, and synaptic function at the Drosophila NMJ.

Involvement of PP2A in viral and cellular transformation

Although the small DNA tumor virus SV40 (simian virus 40) fails to replicate in human cells, understanding how SV40 transforms human and murine cells has and continues to provide important insights into cancer initiation and maintenance. The early region of SV40 encodes two oncoproteins: the large T (LT) and small t (ST) antigens. SV40 LT contributes to murine and human cell transformation in part by inactivating the p53 and retinoblastoma protein tumor suppressor proteins. SV40 ST inhibits the activity of the protein phosphatase 2A (PP2A) family of serine-threonine phosphatases, and this interaction is required for SV40-mediated transformation of human cells. PP2A regulates multiple signaling pathways, suggesting many possible targets important for viral replication and cell transformation. Genetic manipulation of particular PP2A subunits has confirmed a role for specific complexes in transformation, and recent work implicates the perturbation of the phosphatidylinositol 3-kinase/Akt pathway and c-Myc stability in transformation by ST and PP2A. Mutations in PP2A subunits occur at low frequency in human tumors, suggesting that alterations of PP2A signaling play a role in both experimentally induced and spontaneously arising cancers. Unraveling the complexity of PP2A signaling will not only provide further insights into cancer development but may identify novel targets with promise for therapeutic manipulation.

Protein phosphatase 2A B56alpha during development in the spontaneously hypertensive rat

Mistargeting of the regulatory subunit of protein phosphatase 2A (PP2A), B56alpha is involved in the hyperphosphorylation and desensitization of the D1 dopamine receptor in renal proximal tubules of spontaneously hypertensive rats (SHRs). However, the renal expression of B56alpha before hypertension develops is not known. Therefore, we studied the expression of B56alpha and PP2A activity in the kidney during development in the SHR and its normotensive control, the Wistar-Kyoto (WKY) rat. PP2A B56alpha was expressed in proximal and distal tubules with no differences in the pattern of expression in WKY and SHRs at any age. In brush border membranes of renal proximal tubules, PP2A B56alpha protein was greatest in the immature rats and decreased with development. However, PP2A activity did not change with age. PP2A B56alpha protein and PP2A activity were similar in WKY and SHRs except at 2 weeks when both PP2A B56alpha protein and PP2A activity were higher in SHRs than in WKY rats. The PP2A catalytic subunit co-immunoprecipitated with the D1 receptor in renal proximal tubule cells. It is possible that the increased expression of PP2A B56alpha and increased basal PP2A activity in the young, especially in the SHRs, may serve as a compensatory mechanism in the increased phosphorylation and decreased renal D1 receptor function, including D1-receptor mediated stimulation in renal proximal tubules of SHRs.

Expression of the scaffolding subunit A of protein phosphatase 2A during rat testicular development

Previously, we found that the poly(A)+ RNA of the scaffolding subunit A (alpha isoform) of protein phosphatase 2A (PP2A-Aalpha) was clearly expressed by fetal gonocytes but weakly expressed by adult single (As), paired (Apr), and aligned (Aal) A spermatogonia. The scaffolding subunit A of PP2A (PP2A-A) is the major subunit in the formation of a functional PP2A holoenzyme. In this study, we investigated the expression of PP2A-A during testicular development in more detail using in situ hybridization, immunohistochemistry, and Western blot with testes of rats of various ages from 16 days postcoitum (pc) to adulthood. The expression of PP2A-A was detected in fetal proliferative gonocytes at 16 days pc, declining thereafter during the quiescent period of the gonocytes. From the day of birth to the start of spermatogenesis (Day 4 postpartum [pp]), the number of PP2A-A-immunopositive gonocytes increased. At Day 4 pp, the first A1 spermatogonia appeared along the basement membrane; all were PP2A-A positive. In the adult, PP2A-A was upregulated during the differentiation of the As, Apr, and Aal spermatogonia to the A1 spermatogonia and expressed thereafter by all other spermatogonia. Spermatocytes from the pachytene stage onward and all spermatids in the adult testis also showed clear expression of PP2A-A. In Sertoli cells, PP2A-A was detected during their proliferative period at 19 days pc to 15 days pp. The presence of a functional enzyme was confirmed by the additional detection of the catalytic subunit C of PP2A using Western blot analyses at various ages during testicular development. This apparent pattern of expression of PP2A-A during testicular development suggests that PP2A may play an important role in the proliferation of distinct populations of testicular cells and during meiosis and sperm maturation.

Diversity, developmental regulation and distribution of murine PR55/B subunits of protein phosphatase 2A

Protein phosphatase (PP2A) 2A is a hetero-trimeric holoenzyme that consists of a core dimer composed of a catalytic subunit that is tightly complexed with the scaffolding subunit PR65/A. This core dimer associates with variable regulatory subunits of the PR55/B, PR61/B', PR72/B" and PR93/PR110/B"' families. As PP2A holoenzymes containing PR55/B have been shown to be involved in the pathogenesis of Alzheimer's disease, we characterized the PR55/B family with particular emphasis on its distribution and expression in the brain. We determined the genomic organization of all members of the PR55/B family and cloned their murine cDNAs. Thereby, two novel splice variants of PR55/Bbeta were identified. In addition, Northern blot analysis revealed multiple transcripts for the different PR55 subunits, suggesting a higher variability within the PR55 family. In situ hybridization analysis revealed that all PR55/B subunits were widely expressed in the brain. PR55/Balpha and Bbeta protein expression varies significantly in areas of the brain affected by neurodegenerative diseases such as the hippocampus or cerebellum. At the cellular level, PR55/Bbeta protein expression was confined to neurons, whereas PR55/Balpha was also expressed in activated astrocytes indicating that the PR55 isoforms confer a different function to the holoenzyme complex. As PP2A dysfunction has been demonstrated to contribute to various human diseases, dissecting the PP2A holoenzyme and its particular function in different cell types will assist in the development of novel therapeutic strategies.

Chaperonin assisted overexpression, purification, and characterisation of human PP2A methyltransferase

Protein phosphatase 2A (PP2A) is a ubiquitous phosphatase found in many eukaryotic cell types and is involved in regulating a number of intracellular signalling pathways. Its activity, in turn, is regulated through covalent modification, involving phosphorylation and methylation reactions. The effect of phosphorylation on the activity of the protein is well known, but the effects of methylation have only recently been documented and the mechanistic details of methylation are lacking. Methylation, which occurs on the catalytic subunit of PP2A, is catalysed by PP2A methyltransferase (PP2Amt). Here, we present a method for the large-scale purification of human PP2Amt using an Escherichia coli host, coexpressing the chaperonins GroEL and GroES. Purified PP2Amt was identified by peptide mass mapping using MALD-MS and peptide sequencing using ESI-LC-MS/MS. The CD spectrum indicated that purified PP2Amt was folded, with about one-third of the protein adopting an alpha-helical conformation. Analytical gel filtration estimated the molecular weight to be 34kDa, equivalent to the monomeric form of the protein. Further CD analysis showed that in the presence and absence of the ligand S-adenosylhomocysteine, the thermal denaturation profiles were biphasic. However, the transition midpoints shifted to a higher temperature in the presence of ligand, indicating stabilisation of ligand-bound PP2Amt compared to the apo-form. We also report on the progress made in determining the structure of PP2Amt, using both X-ray crystallography and NMR spectroscopy.

Overexpression of the protein phosphatase 2A regulatory subunit Bgamma promotes neuronal differentiation by activating the MAP kinase (MAPK) cascade

Protein serine/threonine phosphatase 2A (PP2A) is a multifunctional regulator of cellular signaling. Variable regulatory subunits associate with a core dimer of scaffolding and catalytic subunits and are postulated to dictate substrate specificity and subcellular location of the heterotrimeric PP2A holoenzyme. The role of brain-specific regulatory subunits in neuronal differentiation and signaling was investigated in the PC6-3 subline of PC12 cells. Endogenous Bbeta, Bgamma, and B'beta protein expression was induced during nerve growth factor (NGF)-mediated neuronal differentiation. Transient expression of Bgamma, but not other PP2A regulatory subunits, facilitated neurite outgrowth in the absence and presence of NGF. Tetracycline-inducible expression of Bgamma caused growth arrest and neurofilament expression, further evidence that PP2A/Bgamma can promote differentiation. In PC6-3 cells, but not non-neuronal cell lines, Bgamma specifically promoted long lasting activation of the mitogen-activated protein (MAP) kinase cascade, a key mediator of neuronal differentiation. Pharmacological and dominant-negative inhibition and kinase assays indicate that Bgamma promotes neuritogenesis by stimulating the MAP kinase cascade downstream of the TrkA NGF receptor but upstream or at the level of the B-Raf kinase. Mutational analyses demonstrate that the divergent N terminus is critical for Bgamma activity. These studies implicate PP2A/Bgamma as a positive regulator of MAP kinase signaling in neurons.

The Saccharomyces cerevisiae type 2A protein phosphatase Pph22p is biochemically different from mammalian PP2A

The Saccharomyces cerevisiae type 2A protein phosphatase (PP2A) Pph22p differs from the catalytic subunits of PP2A (PP2Ac) present in mammals, plants and Schizosaccharomyces pombe by a unique N-terminal extension of approximately 70 amino acids. We have overexpressed S. cerevisiae Pph22p and its N-terminal deletion mutant Delta N-Pph22p in the GS115 strain of Pichia pastoris and purified these enzymes to apparent homogeneity. Similar to other heterologous systems used to overexpress PP2Ac, a low yield of an active enzyme was obtained. The recombinant enzymes designed with an 8 x His-tag at their N-terminus were purified by ion-exchange chromatography on DEAE-Sephacel and affinity chromatography on Ni2+-nitrilotriacetic acid agarose. Comparison of biochemical properties of purified Pph22p and Delta N-Pph22p with purified human 8 x His PP2Ac identified similarities and differences between these two enzymes. Both enzymes displayed similar specific activities with 32P-labelled phosphorylase a as substrate. Furthermore, selected inhibitors and metal ions affected their activities to the same extend. In contrast to the mammalian catalytic subunit PP2Ac, but similar to the dimeric form of mammalian PP2A, Pph22p, but not Delta N-Pph22p, interacted strongly with protamine. Also with regard to the effects of protamine and polylysine on phosphatase activity Pph22p, but not Delta N-Pph22p, behaved similarly to the PP2Ac-PR65 dimer, indicating a regulatory role for the N-terminal extension of Pph22p. The N-terminal extension appears also responsible for interactions with phospholipids. Additionally Pph22p has different redox properties than PP2Ac; in contrast to human PP2Ac it cannot be reactivated by reducing agents. These properties make the S. cerevisiae Pph22p phosphatase a unique enzyme among all type 2A protein phosphatases studied so far.

Protein phosphatase 2A holoenzyme assembly: identification of contacts between B-family regulatory and scaffolding A subunits

Protein serine/threonine phosphatase (PP) 2A is a ubiquitous enzyme with pleiotropic functions. Trimeric PP2A consists of a structural A subunit, a catalytic C subunit, and a variable regulatory subunit. Variable subunits (B, B', and B" families) dictate PP2A substrate specificity and subcellular localization. B-family subunits contain seven WD repeats predicted to fold into a beta-propeller structure. We carried out mutagenesis of Bgamma to identify domains important for association with A and C subunits in vivo. Several internal deletions in Bgamma abolished coimmunoprecipitation of A and C subunits expressed in COS-M6 cells. In contrast, small N- and C-terminal Bgamma deletions had no effect on incorporation into the PP2A heterotrimer. Thus, holoenzyme association of B-family subunits requires multiple, precisely aligned contacts within a core beta-propeller domain. Charge-reversal mutagenesis of Bgamma identified a cluster of conserved critical residues in Bgamma WD repeats 3 and 4. Acidic substitution of paired basic residues in Bgamma (RR165EE) abolished association with wild-type A and C subunits, while fostering incorporation of Bgamma into a PP2A heterotrimer containing an A subunit with an opposite charge-reversal mutation (EE100RR). Thus, binding of A and B subunits requires electrostatic interactions between conserved pairs of glutamates and arginines. By expressing complementary charge-reversal mutants in neuronal PC6-3 cells, we further show that holoenzyme incorporation protects Bgamma from rapid degradation by the ubiquitin/proteasome pathway.

Peptide dendrimers: applications and synthesis

Peptide dendrimers are radial or wedge-like branched macromolecules consisting of a peptidyl branching core and/or covalently attached surface functional units. The multimeric nature of these constructs, the unambiguous composition and ease of production make this type of dendrimer well suited to various biotechnological and biochemical applications. Applications include use as biomedical diagnostic reagents, protein mimetics, anticancer and antiviral agents, vaccines and drug and gene delivery vehicles. This review focuses on the different types of peptide dendrimers currently in use and the synthetic methods commonly employed to generate peptide dendrimers ranging from stepwise solid-phase synthesis to chemoselective and orthogonal ligation.

Genes differentially expressed in the kindled mouse brain

Kindling involves long-term changes in brain excitability and is considered a model of epilepsy and neuroplasticity. Differentially expressed genes in the kindled mouse brain were screened using an reverse transcription-polymerase chain reaction (RT-PCR) differential display (DD) method. C3H male mice were kindled with 40 stimuli in the hippocampus at 5-min intervals. Hippocampal RNA was isolated for DD from mice at 0.5 h, 1 day, 1 week, and 1 month after kindling and from sham-operated controls. About 30,000 bands were screened and of these, 50 were displayed differentially. Northern blot analysis confirmed that 26 of the 50 bands were differentially expressed following rapid kindling. Further sequence analysis revealed that 14 of the genes were previously identified and 12 were novel. The novel genes are referred to as King (1-12) genes because of their association with kindling. According to their temporal and quantitative pattern of expression in forebrain, the 26 genes were grouped into five types. Expression of five of the DD genes, one from each expression type, was further analyzed in hippocampus, forebrain, brainstem, and cerebellum of the kindled mice. Differential expression of these genes was observed in hippocampus and forebrain, but not in brainstem or cerebellum. Only one gene, a regulator of G-protein signaling 4 (RGS4), showed prolonged changes in expression in response to kindling. Our results show that rapid kindling produces spatial and temporal changes in gene expression that may influence kindling-associated neuroplasticity.

The role of adenovirus E4orf4 protein in viral replication and cell killing

It has only been within the last few years that insights have been gained into the remarkable diversity of functions of the adenovirus early transcription region 4 (E4) products. The polypeptide encoded by E4 open reading frame 4 (E4orf4) has emerged as an enigmatic product. Although it accomplishes certain functions that propel viral replication, it has also been shown to be highly toxic, an effect that could dampen the infectious cycle, but that also might serve to facilitate release of viral progeny. When expressed alone, E4orf4 induces a novel form of p53-independent apoptosis in cancer cells but not in normal human cells, thus making it of potential use in cancer gene therapy. In addition, knowledge of its mechanism of action, especially with regard to its interaction with protein phosphatase 2A (PP2A), could provide insights to develop new small molecule anti-cancer drugs. Thus future studies on E4orf4 should be both informative and potentially valuable therapeutically. In this study we review the current status of knowledge on E4orf4.

Reduced expression of the Aalpha subunit of protein phosphatase 2A in human gliomas in the absence of mutations in the Aalpha and Abeta subunit genes

Protein phosphatase 2A (PP2A) consists of 3 subunits: the catalytic subunit, C, and the regulatory subunits, A and B. The A and C subunits both exist as 2 isoforms (alpha and beta) and the B subunit as multiple forms subdivided into 3 families, B, B' and B". It has been reported that the genes encoding the Aalpha and Abeta subunits are mutated in various human cancers, suggesting that they may function as tumor suppressors. We investigated whether Aalpha and Abeta mutations occur in human gliomas. Using single strand conformational polymorphism analysis and DNA sequencing, 58 brain tumors were investigated, including 23 glioblastomas, 19 oligodendrogliomas and 16 anaplastic oligodendrogliomas. Only silent mutations were detected in the Aalpha gene and no mutations in the Abeta gene. However, in 43% of the tumors, the level of Aalpha was reduced at least 10-fold. By comparison, the levels of the Balpha and Calpha subunits were mostly normal. Our data indicate that these tumors contain very low levels of core and holoenzyme and high amounts of unregulated catalytic C subunit.

Toxicity of human adenovirus E4orf4 protein in Saccharomyces cerevisiae results from interactions with the Cdc55 regulatory B subunit of PP2A

The E4orf4 protein of human adenovirus induces p53-independent apoptosis, a process that may promote cell death and viral spread. When expressed alone, E4orf4 kills transformed cells but not normal human cells. The only clear target of E4orf4 in mammalian cells is the Balpha (B55) subunit of protein phosphatase 2A (PP2A), a member of one of three classes of regulatory B subunits. Here we report the effects of E4orf4 in Saccharomyces cerevisiae, which encodes two PP2A regulatory B subunits, CDC55 and RTS1, that share homology with mammalian B and B' subunits, respectively. E4orf4 expression was found to be toxic in yeast, resulting in the accumulation of cells in G2/M phase that failed to grow upon removal of E4orf4. E4orf4-expressing yeast also displayed an elongated cell morphology similar to cdc55 deletion strains. E4orf4 required CDC55 to elicit its effect, whereas RTS1 was dispensable. The recruitment of the PP2A holoenzyme by E4orf4 was entirely dependent on Cdc55. These studies indicate that E4orf4-induced apoptosis in mammalian cells and cell death in yeast require functional interactions with B-type subunits of PP2A. However, some inhibition of growth by E4orf4 was observed in the cdc55 strain and with an E4orf4 mutant that fails to interact with Cdc55, indicating that E4orf4 may possess a second Cdc55-independent function affecting cell growth.

A new role for protein methylation: switching partners at the phosphatase ball

Reversible protein methylation may be another posttranslational modification that serves an important role in modulating signal transduction pathways. Not only does protein phosphatase 2A (PP2A), a key regulator of many signal transduction cascades, undergo this modification, but the methylation process itself may be regulated by various cellular stimuli or states. Mumby describes how PP2A is regulated and how methylation may alter its subcellular targeting and substrate specificity by influencing its interaction with regulatory subunits.

Alterations in protein phosphatase 2A subunit interaction in human carcinomas of the lung and colon with mutations in the A beta subunit gene

Protein phosphatase 2A (PP2A) consists of three subunits, A, B and C. The A and B subunits have regulatory functions while C is the catalytic subunit. PP2A core enzyme is composed of subunits A and C, and the holoenzyme of subunits A, B and C. All subunits exist as multiple isoforms or splice variants. The A subunit exists as two isoforms, A alpha and A beta. Here we report about the properties of eight A beta mutants, which were found in human lung and colon cancer. These mutants were reconstructed by site-directed mutagenesis and assayed for their ability to bind B and C subunits. Two mutants showed decreased binding of PR72, a member of the B" family of B subunits, but normal C subunit binding; two mutants exhibited decreased binding of the C subunit and of B"/PR72; and one mutant showed increased binding of both the C subunit and B"/PR72. Of three mutants that behaved like the wild-type A beta subunit, one is a polymorphic variant and another one is altered outside the binding region for B and C subunits. Importantly, we also found that the wild-type A alpha and A beta isoforms, although 85% identical, are remarkably different in their ability to bind B and C subunits. Our findings may have important implications in regard to the role of PP2A as a tumor suppressor.

PP2A mRNA expression is quantitatively decreased in Alzheimer's disease hippocampus

Since abnormal tau phosphorylation may play a role in neurofibrillary tangle (NFT) formation in aging and Alzheimer's disease (AD), we probed the distribution and abundance of protein phosphatase 2A (PP2A) catalytic (Calpha) and regulatory (PR55alpha and gamma, PR61varepsilon and delta) subunit mRNA in control and AD hippocampus using in situ hybridization. Quantitation of grain density per neuron area of PP2A subunits and beta-actin was determined for the CA3 region of hippocampus and cerebellum, while a qualitative assessment was performed for CA1, CA4, and dentate gyrus. All subunits are expressed in neurons, while PR55gamma and PR55alpha mRNA are also evident in glia. The expression levels of Calpha, all PP2A regulatory subunits studied, and beta-actin were similar in control and AD cerebellum. beta-Actin mRNA was, however, reduced in AD hippocampus. In addition to the generalized reduction of mRNA, as indicated by decreased beta-actin signal, there was a significant loss of Calpha, PR55gamma, and PR61epsilon mRNA in the CA3 hippocampus of AD. This study delineates the distribution of critical PP2A mRNAs and reveals a neuron- and subunit-specific reduction in PP2A catalytic and regulatory mRNA in AD hippocampus. This could result in decreased protein expression and phosphatase activity, leading to the hyperphosphorylation of tau and the formation of NFTs, as well as neuron degeneration in AD.

The Saccharomyces cerevisiae phosphotyrosyl phosphatase activator proteins are required for a subset of the functions disrupted by protein phosphatase 2A mutations

In Saccharomyces cerevisiae, PTPA is encoded by two genes, YPA1 and YPA2. In order to examine the biological role of PTPA as potential regulator of protein phosphatase 2A (PP2A), we compared the phenotypes of the ypaDelta mutants with these of PP2A-deficient strains. While deletion of both YPA genes is lethal, deletion of YPA1 alone results in a phenotype resembling that of PP2A-deficient strains in specific aspects such as aberrant bud morphology, abnormal actin distribution, and similar growth defects under various growth conditions. These phenotypes were even more pronounced when YPA1 was deleted in a pph21Delta genetic background. Moreover, ypaDelta mutants are hypersensitive to nocodazole and show inappropriate mitotic spindle formation as previously described for mutants in the catalytic subunit of PP2A, suggesting that Ypa, like PP2A, has a function in mitotic spindle formation. These results are consistent with an in vivo role of Ypa as a regulator of PP2A. However, unlike a PP2A-deficient strain, ypaDelta mutants do not show a G2 arrest. Therefore, Ypa does not seem to play a role in the regulation of PP2A at this stage of the cell cycle. These results imply that Ypa regulates a specific subset of PP2A functions, possibly by controlling the subunit composition of PP2A.

Protein phosphatase 2A: a highly regulated family of serine/threonine phosphatases implicated in cell growth and signalling

Protein phosphatase 2A (PP2A) comprises a family of serine/threonine phosphatases, minimally containing a well conserved catalytic subunit, the activity of which is highly regulated. Regulation is accomplished mainly by members of a family of regulatory subunits, which determine the substrate specificity, (sub)cellular localization and catalytic activity of the PP2A holoenzymes. Moreover, the catalytic subunit is subject to two types of post-translational modification, phosphorylation and methylation, which are also thought to be important regulatory devices. The regulatory ability of PTPA (PTPase activator), originally identified as a protein stimulating the phosphotyrosine phosphatase activity of PP2A, will also be discussed, alongside the other regulatory inputs. The use of specific PP2A inhibitors and molecular genetics in yeast, Drosophila and mice has revealed roles for PP2A in cell cycle regulation, cell morphology and development. PP2A also plays a prominent role in the regulation of specific signal transduction cascades, as witnessed by its presence in a number of macromolecular signalling modules, where it is often found in association with other phosphatases and kinases. Additionally, PP2A interacts with a substantial number of other cellular and viral proteins, which are PP2A substrates, target PP2A to different subcellular compartments or affect enzyme activity. Finally, the de-regulation of PP2A in some specific pathologies will be touched upon.

Disruption of protein phosphatase 2A subunit interaction in human cancers with mutations in the A alpha subunit gene

The A subunit of protein phosphatase 2A (PP2A) consists of 15 nonidentical repeats. The catalytic C subunit binds to C-terminal repeats 11 - 15 and regulatory B subunits bind to N-terminal repeats 1 - 10. Recently, four cancer-associated mutants of the A-alpha subunit have been described: Glu64-->Asp in lung carcinoma, Glu64-->Gly in breast carcinoma, Arg418-->Trp in melanoma, and Delta171 - 589 in breast carcinoma. Based on our model of PP2A, we predicted that Glu64-->Asp and Glu64-->Gly might be defective in B subunit binding, whereas Arg418-->Trp and Delta171 - 589 might bind neither B nor C subunits. We generated these mutants by site-directed mutagenesis and assayed their ability to associate with different forms of B subunits (B, B', B") or with the catalytic C subunit. The results demonstrate that all mutants are defective in binding either B or B and C subunits. Specifically, the N-terminal mutants, Glu64-->Asp and Glu64-->Gly, are defective in B' but normal in B, B", and C subunit binding, whereas the C-terminal mutants Arg418-->Trp and Delta171 - 589 bind none of the B subunits nor the C subunit. The implications of these findings with regard to the potential role of PP2A as a tumor suppressor are discussed. Oncogene (2001) 20, 10 - 15.

Protein phosphatase 2A: the Trojan Horse of cellular signaling

Dynamic phosphorylation and dephosphorylation of proteins are fundamental mechanisms utilized by cells to transduce signals. Whereas transduction by protein kinases has been a major focus of studies in the last decade, protein phosphatase 2A (PP2A) enzymes emerge in this millenium as the most fashionable players in cellular signaling. Viral proteins target specific PP2A enzymes in order to deregulate chosen cellular pathways in the host and promote viral progeny. The observation that a variety of viruses utilize PP2A to alienate cellular behavior emphasizes the fundamental importance of PP2A in signal transduction. This review will primarily focus on discussing the uniqueness of PP2A regulation and uncovering the critical role played by protein-protein interactions in the modulation of PP2A signaling. Moreover, the place of PP2A in signaling pathways and its functional significance for human diseases will be discussed.

Renal protein phosphatase 2A activity and spontaneous hypertension in rats

The impaired renal paracrine function of dopamine in spontaneously hypertensive rats (SHR) is caused by hyperphosphorylation and desensitization of the renal D(1) dopamine receptor. Protein phosphatase 2A (PP(2A)) is critical in the regulation of G-protein-coupled receptor function. To determine whether PP(2A) expression and activity in the kidney are differentially regulated in genetic hypertension, we examined the effects of a D(1)-like agonist, fenoldopam, in renal cortical tubules and immortalized renal proximal tubule cells from normotensive Wistar-Kyoto rats (WKY) and SHR. In cortical tubules and immortalized proximal tubule cells, PP(2A) expression and activities were greater in cytosol than in membrane fractions in both WKY and SHR. Although PP(2A) expressions were similar in WKY and SHR, basal PP(2A) activity was greater in immortalized proximal tubule cells of SHR than WKY. In immortalized proximal tubule cells of WKY, fenoldopam increased membrane PP(2A) activity and expression of the regulatory subunit PP(2A)-B56alpha, effects that were blocked by the D(1)-like antagonist SCH23390. Fenoldopam had no effect on cytosolic PP(2A) activity but decreased PP(2A)-B56alpha expression. In contrast, in immortalized proximal tubule cells of SHR, fenoldopam decreased PP(2A) activity in both membranes and cytosol but predominantly in the membrane fraction, without affecting PP(2A)-B56alpha expression; this effect was blocked by the D(1)-like antagonist SCH23390. We conclude that renal PP(2A) activity and expression are differentially regulated in WKY and SHR by D(1)-like receptors. A failure of D(1)-like agonists to increase PP(2A) activity in proximal tubule membranes may be a cause of the increased phosphorylation of the D(1) receptor in the SHR.

Dilated cardiomyopathy in transgenic mice expressing a mutant A subunit of protein phosphatase 2A

The protein phosphatase 2A (PP2A) holoenzyme consists of a catalytic subunit, C, and two regulatory subunits, A and B. The PP2A core enzyme is composed of subunits A and C. Both the holoenzyme and the core enzyme are similarly abundant in heart tissue. Transgenic mice were generated expressing high levels of a dominant negative mutant of the A subunit (A delta 5) in the heart, skeletal muscle, and smooth muscle that competes with the endogenous A subunit for binding the C subunit but does not bind B subunits. We found that the ratio of core enzyme to holoenzyme was increased in A delta 5-expressing hearts. Importantly, already at day 1 after birth, A delta 5-transgenic mice had an increased heart weight-to-body weight ratio that persisted throughout life. Echocardiographic analysis of A delta 5-transgenic hearts revealed increased end-diastolic and end-systolic dimensions and decreased fractional shortening. In addition, the thickness of the septum and of the left ventricular posterior wall was significantly reduced. On the basis of these findings, we consider the heart phenotype of A delta 5-transgenic mice to be a form of dilated cardiomyopathy that frequently leads to premature death.

Induction of p53-independent apoptosis by the adenovirus E4orf4 protein requires binding to the Balpha subunit of protein phosphatase 2A

Previous studies have indicated that the E4orf4 protein of human adenovirus type 2 (Ad2) induces p53-independent apoptosis. We believe that this process may play a role in cell death and viral spread at the final stages of productive infection. E4orf4 may also be of therapeutic value in treating some diseases, including cancer, through its ability to induce apoptosis when expressed individually. The only previously identified biochemical function of E4orf4 is its ability to associate with the Balpha subunit of protein phosphatase 2A (PP2A). We have used a genetic approach to determine the role of such interactions in E4orf4-induced cell death. E4orf4 deletion mutants were of only limited value, as all were highly defective. We found that E4orf4 proteins from most if not all adenovirus serotypes induced cell death, and thus point mutations were introduced that converted the majority of highly conserved residues to alanines. Such mutants were used to correlate Balpha-subunit binding, association with PP2A activity, and cell killing following the transfection of appropriate cDNAs into p53-null H1299 or C33A cells. The results indicated that binding of the Balpha subunit is essential for induction of cell death, as every mutant that failed to bind efficiently was totally defective for cell killing. This class of mutations (class I) largely involved residues between amino acids 51 and 89. Almost all E4orf4 mutant proteins that associated with PP2A killed cancer cells at high levels; however, several mutants that associated with significant levels of PP2A were defective for killing (class II). Thus, binding of E4orf4 to PP2A is essential for induction of p53-independent apoptosis, but E4orf4 may possess one or more additional functions required for cell killing.